1. Field of the Invention
The present invention relates to a device for performing a parametric process comprising a photonic crystal periodic structure.
The present invention relates, moreover, to an optical communication line and an optical communication system comprising such device.
2. Description of the Related Art
A parametric process is a typical process of materials having a non-linearity of the χ2 or χ3 type according to which electromagnetic radiations at predetermined frequencies that propagate in such materials interact with each other for generating electromagnetic radiations at different frequencies from those that have generated them (Amnon Yariv, “Optical Electronics”, Third Edition, 1985, HOLT, REINEHART and WINSTON, pages 227-236).
For example, a parametric process is a process according to which a pump radiation at frequency ωp that propagates in a non-linear material, interacting with a signal radiation at frequency ωs, generates a radiation at frequency ωg.
Typical parametric processes are a difference frequency generation process, according to which ωg=ωp−ωs, a sum frequency generation process, according to which ωg=ωp+ωs, a second or third harmonic generation process, according to which ωg=2ωp or, respectively, ωg=3ωp and the known Four Wave Mixing (FWM) process.
For a parametric process to be efficient, a condition known as phase matching must be met.
However, the phase matching condition is difficult to obtain in materials having non-linearity of the ω2 or ω3 type since such materials are typically dispersive, that is, they have a refractive index variable with the frequency ω.
Methods for achieving the phase matching condition have already been proposed.
S. J. B. Yoo et al. (“Wavelength conversion by difference frequency generation in AlGaAs waveguides with periodic domain inversion achieved by wafer bonding”, Appl. Phys. Letters, Vol. 68, No. 19, May 1996, pages 2609-2611), M. H. Chou et al. (“1.5-μm-band wavelength conversion based on difference-frequency generation in LiNbO3 waveguides with integrated coupling structures”, Optics Letters, Vol. 23, No. 13, July 1998, pages 1004-1006) e M. H. Chou et al. (“Multiple-channel wavelength conversion by use of engineered quasi-phase-matching structures in LiNbO3 waveguides”, Optics Letters, Vol. 24, No. 16, August 1999, pages 1157-1159) describe the use of methods known under the name of “periodic domain inversion” or “poling” that essentially consist in periodically inverting the sign of the non-linearity of a non-linear material (for example, LiNbO3 and AlGaAs) so as to obtain a parametric process of difference frequency generation in condition of quasi-phase-matching.
Recently, moreover, the use of one-dimensional photonic crystal periodic structures (also called photonic band-gap structures) has been proposed for carrying out a second harmonic generation parametric process in phase matching conditions.
A one-dimensional photonic crystal structure typically consists of a periodical alternance of two layers of material having different refractive index. The multiple reflections at the interfaces between the two layers at different refractive index generate constructive and destructive interferences between the transmitted light and the reflected light, and make the photonic crystal structure allow the propagation of electromagnetic waves in some intervals of frequencies (or wavelengths) and forbid it in other intervals.
In the present description and claims, the expression                “photonic band gap” or “band gap” is used for indicating a range of frequencies (or wavelengths) that are not transmitted by the photonic crystal structure;        “transmission band” is used for indicating a range of frequencies (or wavelengths) that are transmitted by the photonic crystal structure;        “band gap or transmission band of n-th order” is used for indicating the nth band gap or the nth transmission band as the frequencies increase;        “band edge” is used for indicating the edge of a transmission band;        “nth order low frequency band edge” is used for indicating the edge at the lowest frequency of the transmission band of nth order;        “nth order high frequency band edge” is used for indicating the edge with the highest frequency of the transmission band of nth order.        
G. T. Kiehne et al., (“A numerical study of optical second-harmonic generation in a one-dimensional photonic structure”), Applied Physics Letters, Vol. 75, No. 12, 20 September 1999, pages 1676-1678, present a band-gap engineering approach for obtaining a finite photonic-band-edge-resonant stratified periodic dielectric structure in the presence of material dispersion. When the structure contains second-order nonlinear optical materials, enhanced phase matched optical second harmonic generation may be obtained. The proposed structure consists of four sublayers per period, denoted by (ABCD)N, where N is the number of periods, but only two materials since the dielectric constants are fixed according to the conditions nAC,=n1 and nB,D=n2. The thickness of each layer is defined according to given linear functions of a same parameter α. For the (ABCD)N structure with sufficiently small material dispersion, it is possible to choose α such that doubly resonant second harmonic generation can be obtained. The double-resonance condition is defined to be when both the fundamental frequency and the second harmonic frequency coincide with stop-band edge resonances. The approach is applied to a finite GaAs/AlAs periodic stack. For a fundamental frequency wave of vacuum wavelength of 3.10 μm, a second harmonic wave of wavelength 1.55 μm will be generated in the structure. The approach is not material specific and may be applied as long as the material dispersion is not too large.
M. Scalora et al. (“Pulsed second-harmonic generation in non linear, one dimensional, periodic structures”, Physical Review, Vol. 56, No. 4, October 1997, pages 3166-3174) present a numerical study of second harmonic generation (SH) in a one-dimensional photonic crystal material, doped with a non-linear medium χ2, consisting of 40 dielectric layers (20 periods in all) wherein the refractive index of the layers alternates between n2=1,42857 and n1=1. Moreover, for a reference wavelength λ0, the layers with the two refractive indices n1 and n2 have a thickness a=λ0/4n1 and b=λ0/2n2, respectively. This choice of parameters allows achieving a mixed half-quarter-wave stack for wavelength λ0 and makes the structure have a band gap of the second order removed from the band gap of the first order by approximately a factor 2. This allows realising a second harmonic generation parametric process in which the frequency ω0 of the pump radiation is at the low frequency band edge of the first order and the frequency ωg=2ω0 of the generated radiation is close to the low frequency band edge of the second order. The Authors state that the positioning of the frequencies of the pump radiation and of the generated second harmonic one in the proximity of a band edge allows obtaining a more efficient process since near a band edge a strong overlap between the propagation modes of the pump and of the generated radiation and a co-propagation of the modes occur and the interaction times are larger (“enhancement” phenomenon).
WO 99/52015 describes a second harmonic generator based on a periodic photonic crystal structure. The described structure comprises a plurality of layers of a first and a second material that periodically alternate, and has a band edge at the pump radiation frequency and a second transmission resonance near the band edge of the second order band gap at the generated second harmonic frequency.
M. Centini et al. (“Dispersive properties of finite, one dimensional photonic band gap structures: applications to non linear quadratic interactions”, Physical Review E, Vol. 60, No. 4, October 1999, pages. 4891-4898) discuss the linear dispersive properties of one-dimensional photonic crystal structures consisting of the periodical alternance of two layers with different refractive index and the conditions necessary for optimal second harmonic generation process.
The Applicant notes that one-dimensional photonic crystal structures consisting of the periodical alternance of two layers with different refractive index described by the above documents for realising a second harmonic generation process are not suitable for realising more complex parametric processes (such as, for example, the difference frequency generation process) in phase matching condition.
The Applicant has thus faced the technical problem of providing a device comprising a photonic crystal structure capable of performing a difference frequency generation parametric process in phase matching conditions.
In general, the Applicant faced the technical problem of providing a device comprising a photonic crystal structure that allows performing a parametric process according to which a pump radiation at frequency ωp generates a radiation at frequency ωg by interacting with at least one signal radiation at frequency ωs (with ωs equal to or different from, ωp) in phase matching conditions.
More in general the Applicant faced the technical problem of providing a photonic crystal periodic structure which is suitable to be adapted to perform the above mentioned parametric process.